Aluminum etch process

ABSTRACT

A PROCESS FOR INCREASING THE SURFACE AREA OF ALUMINUM WHEREIN A GALVANIC COUPLE IS FORMED BETWEEN A SURFACE OF THE ALUMINUM AND AN OVERCOATING OF A METAL THAT IS MORE ELECTRONEGATIVE THAT ZINC; THE OVERCOATING IS ACID-WASHED TO FINELY ETCH THE ALUMINUM SURFACE. IN A SPECIFIC EXAMPLE, ZINC IS FIRST DEPOSITED ON THE SURFACE OF HIGH PURITY ALUMINUM FOIL, FROM A SOLUTION OF ZINC OXIDE IN SODIUM HYDROXIDE. COPPER IS THEN DEPOSITED FROM AN AQUEOUS SOLUTION OF CUPRIC CHLORIDE. THE RESULTANT SURFACE IS ACIDWASHED WITH AQUEOUS HYDROCHLORIC ACID CONTAINING A CATALYTIC AMOUNT OF FERRIC CHLORIDE, FOLLOWED BY A NITRIC ACID BATH.

Aug. 17, 1971 H. w. GATES ALUMINUM ETCH PROCESS Filed March 6, 1968 mskmkh rraewtr.;

VUnited States Patent O1 ice 3,600,245 Patented Aug. 17, 1971 U.s. ci. 156-22 s claims ABSTRACT OF THE DISCLOSURE A process for increasing the surface area of aluminum wherein a galvanic couple is formed between a surface of the aluminum and 'an overcoating of a metal that is more electronegative that zinc; the overcoating is acid-washed to finely etch the aluminum surface. IIn a specific example, zinc is first deposited on the surface of high purity aluminum foil, from a solution of zinc oxide in sodium hydroxide. Copper is then deposited from an aqueous solution of cupric chloride. The resultant surface is acidwashed with aqueous hydrochloric acid containing a catalytic amount of ferric chloride, followed by a nitric acid bath.

BACKGROUND OF THE INVENTION (l) Field of the invention This invention pertains to the eld of chemical etching.

(2) Description of the prior art High purity, high surface-area aluminum foil is widely used -in the manufacture of aluminum oxide electrolytic condensers to provide a large capacitance-to-volume ratio and a relatively low cost per microfarad of capacitance.

The capacitance of a condenser is directly proportional to the useful surface area of its plates and efforts have been made to increase the capacitance of aluminum oxide electrolytic condensers by increasing the surface area of the aluminum foil used in their manufacture. These efforts have generally involved electrolytic etching with a salt or halogen acid solution. Salt water etching requires costly current densities, for example, 9000` amperes -for a typical tank and also requires expensive peripheral equipment. Further, the high current density at the surface of the foil as it enters the solution weakens the foil and reduces its tensile strength. Heat generated by the high current opens pores in the aluminum and subsequent cooling entraps salt solution. Subsequent dry heating results in the inclusion of acid-insoluble' crystalline chlorides which decrease the efficiency of the condenser.

IElectrolytic etching with a halogen acid allows smaller current densities to be used; however, currents are still substantial, e.g. 1000 amperes for a moderate size tank. Here also, a reduction in tensile strength is generally experienced.

SUMMARY l01:" THE INVENTION The present invention provides a process Afor increasing the surface area of aluminum without the foregoing drawbacks. The aluminum is finely etched without applied electrical currents and without any significant decrease in tensile strength or entrapment of salts. In one embodiment of the process, a deposit is formed over the aluminum surface of a metal that is more electronegative than zinc. The deposit is then contacted with acid to iinely etch the surface of the aluminum.

It is thought that the metal deposit forms a galvanic couple with the aluminum to generate sufficient voltage to give rise to electrolytic etching without externally applied current. Any acid-soluble metal can be used that will form such a galvanic couple with the aluminum; i.e.

generates greater voltage than generated between zinc and the aluminum. In a particular embodiment, the metal deposited is copper.

In another embodiment of this invention, prior to deposition of the foregoing metal, a different metal is deposited that is more electropositive than the foregoing metal and less electropositive, i.e. more electronegative, than aluminum. Such different metal enables metals less electropositive than itself to be deposited more readily from a salt solution thereof.

In particular embodiments, zinc or tin is first deposited on the aluminum, e.g., from a sodium hydroxide solution thereof, and copper is then deposited, e.g. from any aqueous solution of cupric chloride. The coated aluminum is then contacted with a halogen acid followed by nitric acid.

In another particular embodiment, the acid etching bath contains a catalytic amount of heavy metal salt to reduce the etching temperature.

BRIEF DESCRIPTION OF THE DRAWING The drawing is an essentially diagrammatic sectional view of a tank array and llowsheet depicting the process steps of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, a series of stainless steek tanks is depicted containing `a variety of process solutions and water sprays. At the start of the process, a roll of aluminum foil 10 of high purity, say 99.90% pure, is positioned adjacent a first tank 12. 'Ihe aluminum foil 10 is carried by a flexible plastic leader strap (not shown) threaded over and under polyvinyl chloride rollers 12 positioned within and atop the sides of each tank.

' The foil 10 is carried into the first tank 14 containing a warm aqueous solution of sodium metasilicate 16 acting as degreaser and maintained at about 135-140 F. by a heater 18 submerged in the solution. The aluminum foil 10 is obtained from the rolling mill coated with palm oil which is saponified by the sodium metasilicate. Dilute sodium hydroxide or any other saponier or degreaser may be used in place of the metasilicate.

The foil 10 is next carried into a water-spray tank 20 having nozzles 22 to spray water on the foil 10 and thereby remove the silicate solution. The foil 10 is then carried to a tank 24 containing a dilute solution of nitric acid 26 to ensure the removal of all traces of silicate and from there to another Water spray tank 28 to be washed free of acid solution.

Following the foregoing degreasing and cleansing steps, the aluminum foil 10 is submerged into a tank 30 containing a concentrated water solution of zinc oxide and sodium hydroxide 32. The sodium hydroxide removes any remaining traces of oil or other organic material and the solution 32 strikes the surface of the foil 10, leaving a thin deposit of zinc metal thereon. The zinc does not completely coat the aluminum surface but rather specks of zinc are uniformly and nely dispersed thereover.

The foil 10 is next cleansed in a water-spray tank 34 to remove any cling and is then submerged into a tank 36 lined with rubber 38 and containing an aqueous solution of cupric chloride 40. :In the tank 36, a voltage is immediately generated by the galvanic coupling between the aluminum 10 and the zinc deposit which causes copper metal to plate out.

The copper-coated foil 10 is then submerged into a tank 42 (lined with rubber 44) containing an aqueous solution of hydrochloric acid 46 maintained by a heater 48 therein at about :140 F. Muriatic acid or any other inexpensive commercial grade of hydrochloric acid may be utilized. Removal of copper and etching of aluminum foil 10 occurs immediately and is aided by the voltage generated by the galvanic couple between the aluminum and copper. The hydrochloric acid solution 46 contains a small amount of ferrie ion as ferrie chloride, or other heavy metal ion, which acts as a catalyst for the etching process.

Following etching, the foil 10 is cleansed in a water spray tank 50 and then submerged into a tank 52 containing a concentrated solution of nitric acid S4 which removes residual copper, Zinc and heavy metal. The foil 10 is then washed in a water spray tank 56, and carried successively through three tanks 58, 60 and 62 of de-ionized water 64. It is then submerged into a tank 66 containing a warm dilute solution of boric acid 68, maintained at about 140 F. by a heater 70 submerged therein. Sufficient boric acid adheres to the surface of the foil 10 to leave it slightly acidified.

The foil 10 is next carried past heat lamps 72 where it is dried, `after which it is rolled onto a take-up roll 74. The take-up roll 74 is mounted on a movable shaft 76 responsive to edge sensing means (not shown) as is known in the art, to maintain an even edge on the roll 74.

As a result of the foregoing process, the aluminum foil 10 has a substantially increased surface area. The process is generally useful which any grade of aluminum but is particularly useful with foil that is at least 99.85% pure, that is, having 0.15 weight percent or less of inorganics other than aluminum or aluminum oxide.

Generally, in place of the copper any acid-soluble metal can be used, that will form a galvanic couple with the aluminum to generate a greater voltage than generated between zinc and the aluminum, i.e., a metal that is less electropositive than zinc. The following table sets forth an electrochemical series with the more active metals at the beginning and the more noble metals at the end.

ELECTROCHEMICAL SE RIES Li Se B Mo K Pu Si Sn Rb Th Ta Pb Cs Be Zn Ge Ra U Cr W Ba Hf Ga Cu Sr Al Fc Hg Ca Ti Cd Ag Na Zn In Au La Mn Tl Rh Ce V C Pt Mg Nb Ni Pd Metals at the beginning of the list tend to give up their electrons more readily, that is, they are the most electropositive elements, and will displace later metals from a solution of such later metal. Thus zinc can be first deposited on the foil, as above, then nickel, silver or palladium can be laid thereover followed by etching with acid.

The copper or copper substitute can be applied in the foregoing manner, by submerging the foil in an aqueous solution of its halide or other solution of its ions, e.g. an aqueous solution of cupric sulfate, for a time sufficient for the metal to plate out uniformly over the surface of the aluminum. In general, solutions containing from about 0.1 to about 10 mole percent of such ions can be used. Other methods can also be used, such as electroplating or vapor deposition, but these require expensive peripheral equipment.

In place of zinc one can use generally any metal different from copper or its substitute that is more electropositive than copper or its substitute. Thus, tin can be thinly deposited on the foil and copper laid thereover, followed by acid etching, or cadmium can be deposited on the foil and nickel laid thereover followed by acid etching.

The zinc or zinc substitute can be applied as in the foregoing manner, by dissolving its oxide or other salt in caustic alkali solution, e.g. sodium hydroxide as above, potassium hydroxide, etc., or any appropriate solution, and striking the surface of the aluminum with such solution. Alternatively, it may be deposited by known electrolytic plating methods or by known vapor deposition 4 methods. When using a solution, from about 0.1 mole percent up to a saturated solution of caustic alkali can be used containing from about 0.1 to about 1 mole of metal oxide per mold of caustic alkali.

The foregoing process involves the deposition of two separate metallic layers; the first layer being more electropositive than the second layer. However, the invention has broader applicability in that any method of depositing -the copper or its substitute can be used. For example, a layer of copper can be deposited with the Zinc deposition omitted but with electrolytic contact between the aluminum and cupric chloride. Thus, the aforementioned electrolytic plating methods, or known vapor deposition methods, can be used. The coated aluminum foil can then be acid etched as in the previously described manner. However, the foregoing double-deposition method does facilitate the deposition of the copper or substitute and avoids the use of expensive peripheral equipment and/or high currents required by other methods.

lt was noted that muriatic acid or other commercial or industrial grade of hydrochloric acid can ybe used to etch the aluminum. Generally about to about 80 weight percent of hydrochloric acid can be conveniently used. One can additionally or alternatively use any other acid which will dissolve the copper or its substitute when galvanically coupled with aluminum and which will etch the aluminum. Thus, one can use aqueous sulfuric acid, hydrobromic acid, hydrouoric acid, or -the like, when appropriate.

The addition of ferrie chloride to the acid bath has the effect of lowering the temperature at which rapid etching will occur from about 180 F., in the particular example, to about 140 F. Generally, any heavy metal ion can be used as an etch catalyst; for example, ferrie bromide, nickel chloride, cobalt bromide, and the like can be used to facilitate etching. Generally, the metal has an atomic weight of at least 58 and is preferably a Group VIII metal ion. From about 0.01 to about weight percent is generally effective.

The second acid wash can be nitric acid, as described, or any other strong oxidizing acid that can dissolve residual metal deposits, e.g. hydrouoric acid, hot concentrated sulphuric acid, and the like. In certain cases, where appropriate to the metal, concentrated alkaline solutions may be used.

The final wash with dilute boric acid helps prevent air oxidation and can be replaced by washing with other mild acids such as acetic acid, and the like. Generally, an aqueous solution of from about 1 to about 10 weight percent of such acid is satisfactory.

The following examples will illustrate various aspects of the processes of this invention. i Example 1.-Grade 1194 capacitor aluminum foil, having a purity of 99.94% is processed utilizing a tank array and other equipment as depicted in the drawing and described above. The foil is carried at such a rate that any portion remains submerged in each tank solution for about 40 seconds. It is initially degreased by submersion in a weight percent solution of sodium metasilicate, heated at 140 F. It is washed in a water spray tank, bathed in a 10 weight percent aqueous solution of nitric acid and washed again in a water spray tank.

The foil is then submerged into a near saturated Water solution of sodium hydroxide containing about weight percent, based on the sodium hydroxide, of zinc oxide. It is then washed in a water-spray tank and submerged in a 40 weight percent aqueous solution of cupric chloride, and from there into a bath of 20 Baum muriatic acid containing 2 Weight percent of ferrie chloride, heated at about F.

The foil is then washed in a water spray tank, submerged into a tank of concentrated nitric acid, washed again with a water spray and then bathed in three successive tanks of cle-ionized water. The foil is finally bathed in a weight percent solution of boric acid heated at about 140 F. and lthen dried with heat to yield a finely etched aluminum foil of increased surface area.

In the foregoing Example 1, one may replace the zinc oxide with a similar amount of chrornic sesquioxide hydrate, lead monoxide, gallium sesquioxide hydrate, molybdenum trioxide, tellurium monoxide, tungsten dioxide, or tin dioxide, adjusting the submersion time, when necessary, to obtain a thin deposit of such metal.

One may replace the cupric chloride solution in Example 1 with a similarly concentrated solution of cupric sulphate, lead chloride or nickel chloride. If zinc has rbeen substituted, the copper substitute should be more electronegative than such zinc substitute.

yOne may replace the ferric chloride with a similar amount of another heavy metal halide, eg. nickel chloride, cobaltic chloride, water-soluble chromic chloride or ferrie bromide.

Example 2.-The process of Example 1 is repeated except that the tank of sodium hydroxide-zinc oxide is bypassed and an aqueous solution of weight percent cupric sulphate and 8 weight percent of sulphuric acid is substituted for the cupric chloride solution. A copper electrode is immersed into the cupric sulphate solution and a current of several amperes is applied across the copper electrode as anode and aluminum foil as cathode, to plate copper onto the aluminum foil. The process is then completed as before to etch the aluminum.

Example 3,-The process of Example 1 is repeated except that the sodium hydroxide solution of zinc oxide is omitted and the zinc is instead deposited by electrolysis. Thus, the aluminum foil is passed to an aqueous solution of about weight percent zinc sulfate and about 10 volume percent of sulphuric acid. A zinc electrode is immersed into the solution and current of several amperes is applied across the zinc electrode as anode and aluminum foil as cathode to plate zinc onto the aluminum foil. The process is then completed as before to etch the aluminum.

Example 4.--Aluminum foil is etched by the process of Example l except that a tank of concentrated sulphuric acid replaces the muriatic acid bath, water spray thereafter and nitric acid bath.

Example 5.-Aluminum foil is etched by the process of Example 1 except that the ferrie chloride is omitted from the muriatic acid bath, but its temperature is raised to about 180 F.

It will be understood that other modifications and variations may be effected without departing from the scope of the novel concepts of this invention.

What is claimed is:

1. A continuous process for increasing the surface area of high purity aluminum, comprising:

cleaning said surface to remove organic impurities therefrom;

contacting said clean surface with an alkaline solution of Zinc salt whereby thel deposit Zinc thereon; then applying to said surface a copper salt solution whereby to deposit copper over said surface;

applying to said copper surface an aqueous acid selected from hydrochloric acid, sulfuric acid, hydrobromic acid and hydrofluoric acid, whereby to finely etch said surface; and

washing said surface.

2. The process of clairn 1 wherein said aqueous acid is hydrochloric acid.

3. The process of claim 1 wherein said aluminum is at least 99.85 percent pure prior to said deposition.

4. The process of claim 1 `wherein said acid comprises a halogen acid.

5. The process of claim 1 wherein said acid contains an amount of heavy metal salt dissolved therein sufficient to accelerate said acid etch.

6. The process of claim 1 wherein said copper salt solution comprises an aqueous solution of cupric chloride.

7. The process of claim 1 wherein said aqueous acid is a halogen acid containing an amount of heavy metal salt sufficient to accelerate said acid etch.

8. The process of claim 1 wherein said copper is deposited from an aqueous solution of cupric chloride, said aqueous acid is hydrochloric acid containing an amount of ferric chloride suflicient to accelerate said acid etch and nitric acid is applied to the resultant surface.

References Cited UNITED STATES PATENTS 2,336,846 12/1943 Clark 156-22X 2,681,402 6/1954 Muller 156--22X 2,654,701 10/ 1953 `Calderon et al 15 6-22X JACOB H. STEINBERG, Primary Examiner U.S. Cl. X.R. 156-345; 252-793 

